1. Field of the Invention
The present invention relates to an ink pumping apparatus for printing press having ink pumps for supplying ink fountain roller of an inking mechanism having an ink rail with ink via the ink rail, and more particularly to an ink pumping apparatus for printing press having ink pumps each of which alternately sucks and discharges ink by reciprocating in the axial direction a plunger having a cut portion while rotating it, driven by a stepping motor, and an ink leak preventing method therefor.
2. Description of the Related Art
Ink pumping apparatus for printing press belonging to this technical field include those disclosed in, for instance, Patent Document 1 (Japanese Patent No. 2864447) and Patent Document 2 (Japanese Patent Application Laid-Open No. 2002-349424).
Each of the ink pumping apparatus for printing press disclosed in these Patent Documents (hereinafter referred to as simply “ink pumping apparatus(es)”) has at least one ink pump, and this ink pump is so configured as to alternately accomplish a step of sucking and a step of discharging ink, once each at a time, while a plunger having a cut portion linked to an arm turned by a stepping motor once reciprocates in the main hole of a cylinder while making one turn.
Thus the plunger, while forcing its way into the main hole during its first 180 degrees of a turn, discharges ink through its discharge port. After that the plunger, while moving in the direction of getting out of the main hole during its second 180 degrees of the turn, sucks ink through its intake port. The apparatus further has detecting means which detects every arrival of the turning arm in a prescribed position, and can detect any abnormality in the operation of the pump from the advance or delay in the cycle of detection.
Further, in each of the ink pumping apparatus disclosed in the two Patent Documents, eight ink pumps and a driving motor are incorporated into a base to constitute one pump unit. The intake port of each ink pump is piped to an ink tank via an ink feed passage disposed in the base. Pressurized ink is fed from the ink tank into the ink feed passage. On the other hand, the discharge port of each ink pump is piped to the ink rail for feeding ink to the ink fountain roller.
In the ink sucking process, the plunger blocks the discharge port with its outer circumference except the cut portion, and sucks the pressurized ink into the main hole through the intake port. In the ink discharge process, the plunger blocks the intake port with the said outer circumference, discharges the ink in the main hole through the discharge port and through the ink outlet of the ink rail, and thereby feeds ink to the circumferential face of the ink fountain roller disposed close to the ink outlet.
Each of the stepping motors driving the ink pumps is driven to turn at a different speed from the others during the printing process according to an image area ratio, printing speed and other factors, and stopped at a stop instruction.
Therefore, when the ink pumps stop at a stop instruction, the plungers stop in disorderly phases of rotation, and the outer circumferences of some plungers except the cut portions block either of the intake port and the discharge port while those of others block both when they stop.
The ink pumping apparatus described above involve the following problems to be solved. In those disclosed in Patent Document 1 and Patent Document 2, the plunger of each ink pump is snapped into the main hole with a slight gap so that it can turn and shift in the axial direction within the main hole. Therefore, when the plunger stops with its outer circumference blocking the intake port, the pressure of the ink fed from the ink tank under pressure pushes the outer circumference blocking the intake port, the plunger is deformed within the main hole correspondingly to the gap, and the pressure ink invades into the main hole through the gap, now expanded about twice as wide, between the outer circumference of the plunger and the inner face of the main hole.
Then, if even a very small fraction of the cut portion of the plunger faces the discharge port, the ink having invaded into the main hole further invades into the discharge port through the cut portion, and gradually leaks out through the ink outlet of the ink rail piped to the discharge port. The longer the idle period of the ink pumping apparatus, the greater the quantity of the ink leak.
If the idle period is relatively short, this will invite shifting of an excessive quantity of ink to the circumferential face of the ink fountain roller, disposed close to the ink outlet of the ink rail to match the ink outlet. Or if the idle period is long, the ink leaking through the ink outlet will drip into the external periphery in addition to the excess supply to the ink fountain roller.
Therefore, if the next printing is performed in this state, an excessive quantity of ink will be fed to the form plate in the initial stage of printing, printing will be done at an inappropriately high concentration of ink for some time, and many sheets will be wasted by faulty printing, inviting a corresponding increase in running cost. Moreover, it will be necessary to clear the printing press of the ink having leaked out through the ink outlet during the idle period, imposing an extra load on the staff.